The subject matter disclosed herein relates to turbines and, in particular, to predicting whether a portion of the turbine rotor will rub against the components mounted to the turbine housing or the turbine housing itself during a turbine start.
Turbomachines generally have a centrally disposed rotor that rotates within a stationary housing or shell. A working fluid (gas or steam) flows through one or more rows of circumferentially arranged rotor blades that extend radially from the periphery of the rotor shaft and one or more rows of circumferentially arranged stator blades that extend centripetally from the interior surface of the housing to the rotor shaft. The working fluid interacts with the blades and cause rotation of the rotor shaft to drive a load, such as an electric generator or a compressor. In order to ensure that as much energy as possible is extracted from the fluid, the tips of the stator blades are usually very close to the surface of the rotor. Similarly, the tips of the rotor blades are usually very close to the internal surface of the housing. Such a configuration prevents excessive amounts of working fluid from bypassing the row of rotor blades without imparting energy to them.
Differential thermal expansion during operating conditions between the housing and the rotor results in variations in the tip clearances. In extreme cases, the variations can lead to a phenomenon referred to as “rub” where the tips come into contact with another object. For example, the tips of the rotor blades can contact the housing. Also turbomachines are subjected to a variety of forces under various operating conditions, particularly during transient conditions, such as start-ups, shutdowns, and load changes. These forces may also cause rub. In some cases, rub can damage rotor and stator blades and seals of the turbomachine. The damage leads to costly repairs and machine downtime. Further, when rub is experienced during start up, standard procedures require that the turbomachine be shutdown and then restarted at a later time. This process can lead to further machine downtime.